This invention relates to an improved white luminescent mixed phosphor for use in a cathode ray tube.
For preparation of a white luminescent fluorescent screen of a cathode ray tube for terminal display such as a display tube, because of the absence of a practically available single white luminescent fluorescent phosphor, it has been practiced in the art to employ a mixture of several phosphors with different luminescent colors at appropriate proportions.
Examples of such white luminescent mixed phosphors known in the art may include a mixture of a blue-emitting silver-activated zinc sulfide phosphor (Zns:Ag) and a copper-activated zinc-cadmium sulfide phosphor {(Zn,Cd)S:Cu} emitting light with a complementary color thereof mixed at a suitable proportion so as to emit substantially white color by electron beam excitation (disclosed in U.S. Pat. No. 3,891,886), or a mixture of the three kinds of phosphors of the above blue-emitting ZnS:Ag, a green-emitting manganese-arsenic-activated zinc silicate phosphor (Zn.sub.2 SiO.sub.4 :Mn,As) and a red-emitting manganese-activated zinc phosphate phosphor {Zn.sub.3 (PO.sub.4).sub.2 :Mn} or manganese activated zinc-magnesium phosphate phosphor {(Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn} mixed at a suitable proportion so as to emit substantially white color by electron beam excitation (disclosed in National Technical Report 25, 251 1978).
However, in the case of the former mixed phosphor, since its 10% afterglow period is less than 10 milliseconds (ms), the cathode-ray tube having its fluorescent screen constituted of this mixed phosphor cannot be free from the drawback of greater flicker on its picture screen.
In the case of the latter mixed phosphor, while the flicker on the fluorescent screen can be alleviated due to the longer afterglow period of each of Zn.sub.2 SiO.sub.4 :Mn,As and Zn.sub.3 (PO.sub.4).sub.2 :Mn or (Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn on one hand, there is involved the drawback that no sufficient luminance can be obtained, on the other hand.
For solving such a problem, the present inventors have proposed a mixed phosphor employing, in place of the red-emitting component in the latter mixed phosphor, an orange-emitting phosphor, for example, a manganese activated cadmium chlorophosphate {Cd.sub.5 Cl(PO.sub.4).sub.3 :Mn} phosphor. This mixed phosphor has sufficient luminance and is also excellent in flicker alleviating effect.
However, in this mixed phosphor, since a harmful element of cadmium is contained in Cd.sub.5 Cl(PO.sub.4).sub.3 :Mn as shown above as an exemplary orange-emitting phosphor, it cannot be stated to be preferable from the viewpoint of preventing pollution. Also, in the case of this mixed phosphor, the matrices for the respective component phosphors to be mixed are entirely different materials, and therefore it is difficult to obtain a homogeneous mixture by mixing these componente. Accordingly, a fluorescent screen constituted of this mixed phosphor had the drawback that irregularity is liable to be formed in the color emitted.
On the other hand, there is known a phosphor principally comprised of InBaO.sub.3 and activated with any one of Tb, Eu, Dy and Sm (disclosed in U.S. Pat. No. 3,394,084). However, what is intended in this prior art by the incorporation of these rare earth elements is merely to obtain phosphors of different cathodoluminescence color by incorporation of any one of Tb, Eu, Dy and Sm; for instance, green cathodoluminescence by Tb-activated InBaO.sub.3, or red cathodoluminescence by Eu-activated InBaO.sub.3.